In a remote valley in California, a unique scientific experiment is unfolding.

Drive past the town of Paso Robles - where an earthquake killed two people last Christmas - pass on a few miles from the village of Parkfield (population 18) which has been hit six times in the last century, and follow a narrow dirt track leads through cattle country.

Towering over the arid fields is a drilling rig, normally used to hunt for oil, and now being used to hunt for clues about earthquakes.

In a $20m project funded by the US National Science Foundation (NSF), researchers are aiming to drill directly into the heart of the San Andreas fault.

It would be nice to know when the next one's coming

Gloria Van Horn, Parkfield resident

The main shaft has already reached a depth of about one and half miles (2.5km) and the drill-bit has now been turned to run at an angle, to penetrate the fault from the side.

Nothing so ambitious has ever been attempted before and the excitement among the scientists is palpable.

Where it's happening

The aim of the Safod (San Andreas Fault Observatory at Depth) project is to gather as much data as possible before, during and after earthquakes.

UNDERGROUND OBSERVATORY

Project will drill directly into San Andreas quake zone

It will measure changes in rock before, during, after tremors

Data will be compared with other surface measurements

In the first phase of the drilling, samples of rock are brought to the surface and analysed for their composition and for the traces of gases that swirl around in the depths.

Later this year, when drilling stops for the winter, an array of instruments will be lowered into the hole to provide the clearest possible signals when tremors strike.

One researcher, Stanford University's Naomi Boness, is from the UK. She said: "If we're ever going to move towards better hazard awareness, hazard prediction, then we really need to know a lot more about the fault zone itself, the structure and the composition of the fault zone.

"And this is really a unique and pioneering project because it's the first to directly make those measurements in the fault where the earthquakes are happening."

Her task and that of her colleagues is to build up the best possible picture of activity underground "to try to fit together the pieces of the jigsaw", as she puts it, while sorting samples of mud and rock.

Good prospects

The site was chosen because the area experiences frequent minor quakes - generating the maximum possible amount of data for the scientists.

Indeed Parkfield, the nearest settlement, styles itself the "Earthquake capital of the world".

In the local cafe, selling T-shirts proclaiming "eat here when the big one happens", longtime resident Gloria Van Horn expressed support for the drilling project.

"It would be nice to know when the next one's coming," she told me. Her daughter Cindy was more worried, though.

"They've used explosives before and they've started earthquakes," she said, concerned that the drilling may trigger tremors.

But back at the drill site, as the burly oilmen lower yet another section of pipe into the bore hole, the project scientists are confident that any risks will be outweighed by the benefits.

The work is due to last 20 years. That may be too long to help predict "the next big one", but if any research is to reveal the dark destructive secrets of earthquakes, it could well be here in these dry California hills.

Safod is part of the NSF's EarthScope initiative to explore the structure, evolution, and dynamics of the North American Continent, as well as the physical processes controlling its earthquakes and volcanic eruptions.